An 80-year-old man with a history of Helicobacter pylori eradication therapy underwent esophagogastroduodenoscopy, and a flat elevated lesion measuring 5 mm with discoloration and vascular dilation was noted on the gastric fornix. Endoscopic biopsy revealed adenocarcinoma of the fundic gland-type (GA-FG), and endoscopic submucosal dissection (ESD) was performed. The pathological diagnosis was GA-FG with superficial submucosal layer invasion. In addition, a 1-mm GA-FG was incidentally detected on the oral side of the resected specimen. This small lesion was hard to visualize even retrospectively. Considering the development pattern of GA-FG, originating in the fundic gland area and covered by nonneoplastic epithelium, there may be undetectable lesions hidden around the visibly detectable GA-FG. Therefore, it is essential to meticulously observe the area around the lesion before treatment and perform regular endoscopic follow-up after treatment.
